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Nicolas Léonard Sadi Carnot (1 June 1796 — 24 August 1832) was a French military engineer who, in his 1824 book Reflections on the Motive Power of Fire, gave the first successful theoretical account of heat engines, now known as the Carnot cycle; his book also laid the foundations for the second law of thermodynamics. He is often described as the "Father of thermodynamics", being responsible for such concepts as Carnot efficiency, Carnot theorem, the Carnot heat engine, and others.

A Carnot heat engine[2] is a hypothetical engine that operates on the reversible Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824. The Carnot engine model was graphically expanded upon by Benoît Paul Émile Clapeyron in 1834 and mathematically elaborated upon by Rudolf Clausius in the 1850s and 60s from which the concept of entropy emerged.

Every thermodynamic system exists in a particular state. A thermodynamic cycle occurs when a system is taken through a series of different states, and finally returned to its initial state. In the process of going through this cycle, the system may perform work on its surroundings, thereby acting as a heat engine.

A heat engine acts by transferring energy from a warm region to a cool region of space and, in the process, converting some of that energy to mechanical work. The cycle may also be reversed. The system may be worked upon by an external force, and in the process, it can transfer thermal energy from a cooler system to a warmer one, thereby acting as a refrigerator or heat pump rather than a heat engine.

In the adjacent diagram, from Carnot's 1824 work, Reflections on the Motive Power of Fire,[3] there are "two bodies A and B, kept each at a constant temperature, that of A being higher than that of B. These two bodies, to which we can give or from which we can remove the heat without causing their temperatures to vary, exercise the functions of two unlimited reservoirs of caloric. We will call the first the furnace and the second the refrigerator.”[4] Carnot then explains how we can obtain motive power, i.e. “work”, by carrying a certain quantity of heat from body A to body B.